1) Not all girls expect their bf to pay for their everything.
Some will get angry if their bf tries to, it makes them look as
though they can't afford it. Girls have ego too you know?
2) Can always say I'm going through a tight period/ saving up
for something. So you don't wish to spend a lot of money and hope
she understands. If she suggest going to a fancy restaurant, ask
her if kopitiam can? Saving money is a virtue. All in all, be
honest and straight to the point.
3) Can refer to point 2. Otherwise, can try suggesting a home
cook meal too. Every time money runs out, head home for dinner.
Just an advice, next time when start a r/s, please do not set the
standard by visiting expensive restaurants and buying gifts
regularly. If you know you cannot keep it up, don't make it a habit
for her. We guys have our pride, but still, a reflection on our
actual character is what we would want to show the
girl. 
4) Through observation? I usually go for girls who are frugal. Her
money, My money, both also cannot spend that sort. 